A DI can is a two-piece can having no joint between the trunk and wall, and is produced by drawing (cupping) a metal sheet to form a cup, followed by ironing, or redrawing and ironing. DI cans are used for packing beverages such as beer and soft drinks, and foods such as soups and vegetables.
In the drawing operation, a disk stamped out from a metal sheet is held by a blank holder in a drawing machine called cupping press, and formed into a cup with a tool composed of a punch and a die. In the ironing operation, the side wall of the drawn cup is stretched into a thin section.
In the drawing operation, if the diameter of the disk-shaped metal sheet is too larger than the diameter of the ironing punch, a cup having a desired shape may be not formed by one time of the drawing operation. In this case, the drawing operation is usually carried out twice (drawing-redrawing) thereby forming a desired shape. Through the operations, a cup having a relatively larger diameter is produced by the cupping press, the cup is redrawn in a body maker (can body former), and then ironed.
DI cans are usually made of a metal sheet such as a tin-plate or aluminum sheet. The metal sheet is formed into a desired shape by the DI forming, and subjected to aftertreatment such as washing, surface treatment, and coating, thus producing a DI can. In recent years, with the aim of omitting or simplifying the aftertreatment, a method for producing a DI can from a film-laminated metal sheet (hereinafter may be referred to as a laminated metal sheet) is studied thereby producing a can without aftertreatment.
When a film-laminated metal sheet is formed into a two-piece can body by the DI forming, the film must have very high processability so as not to be perforated or peeled from the metal substrate during ironing. Further, when the laminated metal sheet is formed into a two-piece can body for packing a food, the can body will be exposed to harsh conditions such as retort treatment. Therefore, the film must have durability after a high level of processing.
As described above, when a laminated metal sheet is formed by the DI forming into a two-piece can body for packing a food, the film must have very high processability and durability. Although studies have been made on laminated metal sheets for making DI cans for beverages used in moderate environments, there has been no laminated metal sheet resistant to high levels of processing and harsh conditions.
For example, Patent Documents 1 to 4 describe coating films for metal sheets. The films, which are composed of butylene terephthalate and ethylene terephthalate, are laminated principally to aluminum sheets, and subjected to drawing and ironing. However, the smooth laminated metal sheets described in Patent Document 1 to 4 tend to have film defects or deteriorate in their adhesion to the metal substrates during the DI forming, and thus likely cannot endure harsh conditions such as retort treatment when used to make a food can. In particular, when the substrate is a steel sheet having a higher strength than an aluminum sheet, the coating film tends to be damaged during forming, and thus may be unserviceable under harsh conditions.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-88233    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-335682    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-58402    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2004-249705
That is to say, there has been no method for making a highly processed two-piece can body such as a DI can from a laminated metal sheet with durability to harsh conditions such as retort treatment.
The present invention has been made in view of the above circumstances, and is intended to provide a laminated metal sheet suitable for making a highly processed two-piece can body resistant to harsh conditions such as retort treatment, and a two-piece can body made of the laminated steel sheet.